1. Field of the Invention
The present invention relates to an output driver of a semiconductor memory device, and more particularly to an output driver of a semiconductor memory device capable of operating both in a differential mode and in a single mode.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a conventional output driver.
The output driver of FIG. 1 has a circuit configuration of a differential amplifier and may be used in a system operating at a high frequency of over 800 MHz. According to FIG. 1, when a positive input signal VIP has a logic high state and a negative input signal VIM has a logic low state, a positive output signal VOP has a logic high state and a negative output signal VOM has a logic low state. P-type metal-oxide semiconductor (PMOS) transistors MP1 and MP2 supply a current to an output driver of FIG. 1 in response to a bias voltage VB1. An n-type MOS (NMOS) transistor MN3 is coupled between a node N1 and a low supply voltage and supplies a constant current to the output driver in response to a bias voltage VB2. That is, the NMOS transistor MN3 operates as a current source.
Most function tests, such as those other than a test of input/output the number of pins in a semiconductor device used in the test increases, the number of semiconductor devices that can be tested using a tester in a test system at one time can likewise decrease. During a low frequency test, the output driver does not need to operate in a differential mode, but rather can be operated in a single mode, since the high-frequency characteristics of differential mode are not needed for low frequency tests. When the output driver operates in a single mode, the number of pins that is used in the test may be decreased. A conventional output driver does not take advantage of this.